Time domain differential techniques to characterize various stimuli

ABSTRACT

A method for determining a stimulus is provided. The method includes determining a touch condition based on the rate of change of electrode capacitance, measuring a characteristic of the electrode capacitance in response to the touch condition being met, and evaluating the measured characteristic to determine the touch stimulus. The method can improve the ability to determine a touch stimulus over existing methods, including the ability to determine fingerprint and handprint biometrics, for example.

BACKGROUND OF THE INVENTION

The present invention relates to methods for characterizing variousstimuli, for example touch inputs, using time domain differentialsensing techniques.

Touch inputs are widely used as an input methodology. For example, touchinputs are used in conjunction with appliances, tablets, andsmartphones. Touch inputs are also used in conjunction with fingerprintsensors. For example, fingerprint swipe sensors can use conventionalcapacitive sensing techniques to convert a touch swipe into atwo-dimensional image of a fingerprint. Handprint sensors are anextension of this technology, and can generate a pixelated image whereeach pixel includes a greyscale value proportional to the distance fromthe skin.

Touch inputs can be determined based on a capacitive output of anelectrode. According to one known method, the value of the capacitiveoutput is used to determine the presence of a touch input on asubstrate, the location of a touch input on a two-dimensional panel, orthe individual peaks and valleys in a fingerprint. However, thecapacitive output is generally compared against a reference value, whichin practice is an approximation. The reference value can lead to falsetouch inputs or the failure to register actual touch inputs depending onmanufacturing tolerances, environmental changes, and otherconsiderations.

SUMMARY OF THE INVENTION

Methods for determining a stimulus are provided. The methods generallyinclude detecting a touch condition based on the rate of change ofelectrode capacitance and evaluating the touch condition after itsdetection. The methods can conserve computing resources by deferring theevaluation of the touch condition until after the touch condition hasbeen detected, while also providing additional data, in particular rateof change data, for evaluating the touch condition.

According to one embodiment, a method includes determining a touchcondition based on the rate of change of electrode capacitance,measuring a characteristic of the electrode capacitance in response tothe touch condition being met, and evaluating the measuredcharacteristic to determine the touch stimulus. The method is adaptedfor use with capacitive sensors, including touch screens, touch pads,fingerprint sensors, and handprint sensor, and can be further modifiedfor use with optical sensors.

In one embodiment, determining a touch condition includes determiningthe rate of change of electrode capacitance. The rate of change ofelectrode capacitance can decrease, slowing to nearly zero, as an objectcomes to rest against a touch surface. As the rate of change ofelectrode capacitance falls below a threshold value, a touch conditionis registered. The touch condition can correspond to the object comingto rest, or very nearly to rest, for example the placement andflattening of a fingertip against a touch substrate. The touch conditioncan also correspond to placement of a finger against a fingerprintsensor for the subsequent evaluation of a fingerprint.

In one embodiment, measuring a characteristic of the capacitanceincludes measuring the instantaneous capacitance, the rate of change ofcapacitance, or both the instantaneous capacitance and the rate ofchange of capacitance. This step can include detecting individual ridgesand valleys in a fingertip, which is optionally performed only after thetouch condition is met, thereby conserving computing resources.Detecting individual ridges and valleys can include sampling a pluralityof capacitors positioned beneath the fingertip, or sampling a singlecapacitive sensor as a fingertip slides over a capacitive sensor.Measuring a characteristic of the electrode capacitance can also includemeasuring the rate of change of electrode capacitance from prior to,coincident with, or after a touch condition is registered.

In one embodiment, evaluating the measured characteristic includes acomparison with data stored to computer readable memory. For example,the measured characteristic can include a rate of change of electrodecapacitance, and the data stored to computer readable memory can includea look-up table including various stimuli and their corresponding rateof change of electrode capacitance. Also by example, the measuredcharacteristic can include an image of a fingerprint, and the datastored to memory can include previously collected biometric datacorresponding to a plurality of different fingerprints.

The present embodiments can be very valuable when coupled withconventional fingerprint sensing techniques. Using a variety oftechniques, fingerprint sensing is meant to measure fingerprints thatare unique to individuals. Therefore a fingerprint measured shouldcorrespond to an individual and there should be one unique fingerprintmeasurement per person. In order to measure a fingerprint, manytechniques require that the finger be pressed to a surface so thatsensing circuitry can measure the fingerprint. If the finger is abovethe surface or perhaps lightly touching the measurement surface, thenthe sensing circuitry may sense inconsistent inputs. If placement of afinger against the surface is first detected and then the data forfingerprint sensing is obtained, data obtained from the sensingcircuitry can be more consistent for comparing to reference data. Thereference data itself may be more consistent and reliable if the samesensing of touch first and the fingerprint sensing data is then taken aswell.

These and other features and advantages of the present invention willbecome apparent from the following description of the invention, whenviewed in accordance with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating a method for determining a stimulusin accordance with an embodiment of the present invention;

FIG. 2 is a flow chart illustrating a method for determining afingerprint in accordance with an embodiment of the present invention;and

FIG. 3 is a flow chart illustrating a method for determining a stimulusin accordance with a third embodiment of the present invention.

DESCRIPTION OF THE CURRENT EMBODIMENTS

The current embodiments generally relate to methods for determiningvarious stimuli. The methods generally include detecting a touchcondition based on the rate of change of electrode capacitance andevaluating the touch condition after its detection. The methods canconserve computing resources by deferring the evaluation of the touchcondition until after the touch condition has been detected, while alsoproviding additional data, in particular rate of change data and touchstimulus level, for evaluating the touch condition. The methods areadapted for use with a wide variety of capacitive sensors, includingtouch screens, touch pads, fingerprint sensors, and handprint sensors,and are equally applicable to optical sensors, including opticalfingerprint sensors.

The methods of the present invention can be performed by the sensingcircuits set forth in WIPO Publication WO2010/111362 to Caldwell et alentitled “Apparatus and Method for Determining a Touch Input,” and WIPOPublication WO2013/163496 to Caldwell et al entitled “Apparatus andMethod for Determining a Stimulus, Including a Touch Input and a StylusInput,” the disclosures of which are incorporated by reference in theirentirety. Further by example, the embodiments may be implemented incombination with the electrode structure at FIG. 22 of WO2013/163496.Other sensing circuits and other electrodes can be used as desired.

Referring now to FIG. 1, a flow chart illustrating a method inaccordance with a first embodiment is provided. In general terms, amethod for determining a stimulus includes a) applying time domaindifferential techniques to detect a touch condition, b) measuring acharacteristic of the touch condition in response to the touch conditionbeing met, and c) evaluating the measured characteristic to determineinformation regarding the touch stimulus.

Applying time domain differential techniques to detect a touch conditionis depicted as step 10 in FIG. 1. This step generally includes measuringthe rate of change of electrode capacitance as an object approaches atouch substrate. This step can be aided by determining whether theobject is proximate the touch substrate before measuring for a rate ofchange of electrode capacitance. For example, this step can includecomparing a measured electrode capacitance against a setpointcapacitance. When the measured electrode exceeds a setpoint capacitance,the rate of change of electrode capacitance is then determined.Determining the rate of change of electrode capacitance can includedetermining the change in capacitance over time. Where the samplinginterval remains constant, this operation can include dividing thedifference between successive capacitive values over the samplinginterval. The measured rate of change is then compared against athreshold rate of change. If the measured rate of change falls below thethreshold rate of change, the object is determined to have come to rest,or nearly come to rest, with respect to a touch substrate or over atouch substrate. If the measured rate of change then increases fromzero, or nearly zero, the object is determined to have begun recedingfrom its previous position against or over the touch substrate. A touchcondition corresponds to placement of an object against a substrate inthe present embodiment, being at least momentarily at rest, but caninclude movement of an object along a substrate in other embodiments.

Further with respect to step 10 of FIG. 1, additional time domaindifferential techniques are disclosed in WO2010/111362 andWO2013/163496.

Measuring a characteristic of a touch condition is depicted as step 12in FIG. 1. This step generally includes measuring a capacitance ormeasuring a rate of change of capacitance. For example, in embodimentswhere the touch condition includes placement of an object against asubstrate, the characteristic can include the capacitive output of oneor more electrodes to map the individual ridges and valleys in afingertip in accordance with known sensing techniques, for example thesensing techniques disclosed in Advances in Biometrics: Sensors,Algorithms and Systems by Ratha et al (Springer Science & BusinessMedia, Copyright 2008), or U.S. Pat. No. 4,353,056 to Tsikos, thedisclosures of which are hereby incorporated by reference in itsentirety. The one or more electrodes can optionally include a largenumber of small electrodes that are smaller than the width of the ridgesand valleys in order to obtain high resolution of the fingertip. Furtherby example, the characteristic can include the cumulative capacitance ofthe object, which can be used at step 14 for the detection of spoofingattempts using prosthetic fingers. Still further by example, thecharacteristic can include the rate of change of electrode capacitanceprior to, during, and/or after the touch condition, which can also beused at step 14 for the detection of spoofing attempts using prostheticfingers.

Evaluating the measured characteristic to determine a stimulus isdepicted as step 14 in FIG. 1. This step generally includes comparingthe measured characteristic to pre-existing data in computer readablememory. The pre-existing data can include a look-up table with valuesthat may or may not correlate to the measured characteristic. Forexample, where the measured characteristic includes a capacitive imageof the individual ridges and valleys of a fingerprint, the look-up tablecan include a list of biometric values corresponding to the location ofindividual ridges and valleys on a known fingertip. Further by example,where the measured characteristic includes the cumulative capacitancefor a finger, the look-up table can include the cumulative capacitanceof a known finger as determined by the fingerprint match above. In thisregard, the method guards against spoofing attempts using prostheticfingers, which typically provide a measurably different capacitiveoutput, even if the ridges and valleys are a match. Still further byexample, where the measured characteristic includes the rate of changeof capacitance, the look-up table can include the rate of change of afinger as previously collected from one or more measurements. In thisregard, the computer readable memory can include a database of biometricfingerprint data, where each fingerprint match is validated by either orboth of the expected capacitance or the rate of change of capacitance ofthe user's finger, where these values are detected before, during, orafter the touch condition.

To reiterate, any electrode configuration with any time domaindifferential measurements can be used to determine the actual touchcondition. Once the touch condition has been detected, then any of themeasured parameters that are available prior to, substantiallycoincident with, or after the touch condition can be used tocharacterize the touch condition, also referred to as a stimulus(stimuli data). Using this technique, the stimuli data can be used todetect if there is a touch event generated when using a gloved finger oran ungloved finger. This can be used to compare the stimuli data withpreviously stored data whether over a short or long term duration. Also,the stimuli data can be compared with predetermined data stored asconstants and/or references.

Referring to FIG. 2, a flow chart illustrating a method in accordancewith a second embodiment is provided. In general terms, the method forfingerprint recognition includes a) applying time domain differentialtechniques to detect a touch condition, b) measuring fingerprint datausing fingerprint sensing circuitry, and c₁) storing the fingerprintdata for calibration and reference or c₂) comparing the fingerprint datafor proper fingerprint recognition.

Applying time domain differential techniques to detect a touch conditionis depicted as step 20 in FIG. 2. This step is functionally identical tostep 10 of FIG. 1. Measuring fingerprint data is depicted as step 22 inFIG. 2. This step generally includes measuring the biometric data inresponse to the touch condition having been detected at step 20. Thatis, to conserve processing resources, the method includes deferring thecollection of finger biometric data until after the finger hasdefinitively come to rest against the substrate as measured by thedesired sensing circuit. Once the finger has come to rest, the samesensor or a different sensor measures fingerprint data according totechniques now known or hereinafter developed, including the techniquesdisclosed in Ratha et al. The fingerprint data is optionally processedfor relevant biometric markers by a processor electrically coupled tothe sensor performing the fingerprint measurement. Storing thefingerprint data for calibration or reference is depicted as step 24 inFIG. 2. This step generally includes storing the fingerprint data and/orthe biometric markers in computer readable memory for future use.Comparing the fingerprint data is depicted as step 26 in FIG. 2. Thisstep generally includes a comparison against biometric data previouslystored to memory. The biometric data can include all or a portion of thefingerprint data, including any biometric markers that were derived inthe measurement of the fingerprint at step 22.

As noted above, existing fingerprint sensing techniques can be trickedinto misdiagnosing fingerprints. For example, spoofing can beaccomplished by imprinting a forged fingerprint onto a carrier materialthat is then placed on a finger. However, time domain differentialtechniques can be used to detect the carrier material. By first sensingthe touch condition and then evaluating the touch characteristicsavailable prior to, during, and after the touch condition, the method ofthe present embodiment can determine if the touch condition isattributable to a human finger alone or if the touch condition isattributable to a carrier material on a human finger.

Referring to FIG. 3, a flow chart illustrating a method in accordancewith a third embodiment is provided. In general terms, a further methodfor fingerprint recognition includes a) applying time domaindifferential techniques to detect a stimulus, b) measuring a parameterof stimulus strength, c) comparing the stimulus strength to apredetermined threshold corresponding to a bare finger, d) if thestimulus strength is less than the predetermined threshold, register aninvalid fingerprint, e) if the stimulus strength is greater than orequal to the predetermined threshold, measuring fingerprint data usingfingerprint sensing circuitry, and f₁) storing the fingerprint data forcalibration and reference and/or f₂) comparing the fingerprint data forproper fingerprint recognition.

Applying time domain differential techniques to detect a stimulus isdepicted as step 30 in FIG. 3. This step is functionally identical tostep 10 of FIG. 1. Measuring a parameter of stimulus strength isdepicted as step 32 in FIG. 3. For example, if the stimulus is a touchcondition, the parameter can include the magnitude of the capacitivesignal strength corresponding to placement of an object (finger or otherimplement) against a touch surface. Comparing the stimulus strength to apredetermined threshold is shown as step 34 in FIG. 3. This stepgenerally includes a comparison of the magnitude of the signal strengthagainst a predetermined threshold that corresponds to a bare finger.This step is typically performed in digital or analog logic. If thefinger were to include a carrier material, the measured magnitude isexpected to be less than the predetermined threshold, because the humantissue is spaced apart from the touch substrate by a distance equal tothe thickness of the carrier material. Under this scenario, the methodwould return an unverified fingerprint at step 36. If however the fingerdoes not include a carrier material, the measured magnitude is expectedto be greater than or equal to the predetermined threshold. Under thisscenario, the method would continue to measure the fingerprint data atstep 38. This step generally includes measuring fingerprint data forrelevant biometric markers by a processor electrically coupled to asensor performing the fingerprint measurement. Storing the fingerprintdata for calibration or reference is depicted as step 40 in FIG. 3. Thisstep generally includes storing the fingerprint data and/or thebiometric markers in computer readable memory for future use. Comparingthe fingerprint data is depicted as step 42 in FIG. 3. This stepgenerally includes a comparison against biometric data previously storedto memory. The biometric data can include all or a portion of thefingerprint data, including any biometric markers that were derived inthe measurement of the fingerprint at step 22.

As set forth above, the methods of the present invention can beperformed in connection with a time domain differential sensingapparatus, including those set forth in WO2010/111362 and WO2013/163496.The time domain differential sensing apparatus generally performs atleast method steps 10 in FIG. 1, 20 in FIG. 2, and 30, 32 and 34 in FIG.3. The remaining method steps can be performed by a computer apparatushaving a processor to execute a series of commands representing one ormore of the remaining method steps schematically depicted in FIGS. 1-3.The computer apparatus is generally programmed with a series ofinstructions that, when executed, cause the processor to perform methodsteps as described above. The instructions that are performed by theprocessor are generally stored in a computer readable data storagedevice. The computer readable data storage device can be a portablememory device that is readable by the computer apparatus. Such portablememory devices can include a compact disk, a digital video disk, a flashdrive, and any other disk readable by a disk driver embedded orexternally connected to a computer, a memory stick, or any otherportable storage medium whether now known or hereinafter developed.Alternatively, the machine-readable data storage device can be anembedded component of a computer such as a hard disk or a flash drive ofa computer. Together, the computer and machine-readable data storagedevice can be a standalone device or embedded into a machine or a systemthat uses the instructions for a useful result. Additionally, any ofthese techniques can use light emitters, light detectors, and varyinglight as affected by a finger in substitution of electric fieldelectrodes and then using time domain differential sensing processingtechniques.

The above description is that of current embodiments of the invention.Various alterations and changes can be made without departing from thespirit and broader aspects of the invention as defined in the appendedclaims, which are to be interpreted in accordance with the principles ofpatent law including the doctrine of equivalents. Any reference toelements in the singular, for example, using the articles “a,” “an,”“the,” or “said,” is not to be construed as limiting the element to thesingular.

1-21. (canceled)
 22. A method comprising: providing a capacitive sensorhaving an output signal; detecting a placement of an object against atouch surface by calculating a rate of change of the capacitive sensoroutput signal, wherein detecting a placement of an object against atouch surface includes measuring a magnitude of the capacitive sensoroutput signal; in response to detecting placement of the object againstthe touch surface, comparing the magnitude of the capacitive sensoroutput signal with a predetermined threshold capacitance correspondingto the capacitance of skin stored to computer readable memory to verifythe object is skin lacking a carrier material; and in response toverifying the object is skin based on a determination that the magnitudeof the capacitive sensor output is greater than the predeterminedthreshold capacitance, mapping ridges and valleys of the skin using thecapacitive sensor output; wherein the comparing the magnitude of thecapacitive sensor output with the predetermined threshold capacitancecorresponding to the capacitance of skin stored to the computer readablememory occurs prior to the mapping the ridges and valleys of the skinusing the capacitive sensor output.